The invention relates to a method for processing signals, particularly for oximetric measurements on living human tissue, in which spurious signals are suppressed with respect to information signals, said spurious signals having a frequency lying in a first frequency range and said information signals having a frequency lying in a second frequency range being different from said first frequency range, said signals being conducted over a filter having essentially a blocking characteristic in said first frequency range and having essentially a transmission characteristic in said second frequency range, an output signal of said filter being further processed.
It is well-known in the art to perform measurements of various physical quantities by using detectors, to convert the physical quantity into electric voltages. However, when processing the electrical signal, derived from such detectors, one has to take into account that the information signals, i.e. the signals, representing the desired physical quantities, are mostly superimposed by spurious signals coming from various sources. The effect of spurious signals becomes more important, the more the sensitivity of the measuring circuitry is enhanced. Typical examples for spurious signals are drift signals, i.e. low-frequency aberrations being generated by thermal effects, by slow alterations of supplying voltages, etc.
When the frequency of such spurious signals is different from the frequency of the information signals, one can easily suppress the spurious signals by inserting filter circuits into the measuring circuitry which have a blocking characteristic in the frequency range of the spurious signals and which have a transmission characteristic in the frequency range of the information signals.
In the field of oximetric measurements on living human tissue, it is known to use photoelectric probe heads exhibiting a plurality of light-emitting elements, e.g. light-emitting diodes (LED) which are tuned to different wavelengths so that light beams of different wavelength may be emitted on the human tissue under investigation. The light beams, having penetrated the tissue, are then directed on a photo-sensitive device which converts the impinging light beams into electrical signals.
However, when performing such measurements, one potential source of spurious signals is the ambient light at the location where the measurement is performed. Considering that human tissue is partially transparent to light, it can easily be understood that the photo-sensitive device used in oximetric measurements is not only subject to the light beams, generated by the light-emitting elements but also to ambient light, be it generated by electric lamps or be it natural day-light. Ambient light may vary in amplitude during the time where the measurement is performed so that the photo-sensitive device will detect a mixture of slowly varying ambient light and of the light beams generated by the light-emitting elements.
In order to overcome these deficiencies, it is well-known in the art to use multiplex techniques. For this purpose, pulse trains are generated, being composed by individual pulses, each of which being generated by a light-emitting element and, thus, corresponding to a light beam of different wavelength. One can, for example, use pulse trains having three individual pulses corresponding to short light pulses of three different wavelengths. One can, further, timely separate the pulse trains by a short break, during which no light is emitted so that the electrical signal, generated by the photo-sensitive device during such break, is only dependent on ambient light.
As long as the influence of ambient light is constant, one can easily measure an offset-value corresponding to the electrical signal during the break and can subtract the offset-value from any succeeding electrical signals received when the pulse trains appear. Such offset-compensation is, however, only effective if the influence of ambient light is constant within the desired precision of measurement.
However, in most cases, this is not true, because the influence of ambient light varies with time and can, therefore, not be compensated by simply subtraction measures.
Therefore, one has tried to overcome these problems by inserting appropriate filter circuitry into the signal path behind the photo-sensitive element. Considering that the pulse frequency is relatively high, i.e. in the order of magnitude of several hundred cps, and considering, further, that the variation of ambient light is in the order of a few cps, one has used high-pass filter circuits to suppress spurious signals generated by the variation of ambient light.
However, due to the fact that all filters have a frequency characteristic influencing frequency bands lying octaves away, inserting a high-pass filter into the signal path of an oximetric measuring instrument would result in a distortion of the information signal even if the frequency of the information signals is several orders of magnitude away from the spurious signal frequency. This holds true the more the sensitivity of the oximetric system shall be enhanced which requires a high-precision of amplitude measurement on the light pulses received by the photo-sensitive element.
It is, therefore, an object of the present invention to improve the method mentioned above by effectively suppressing spurious signals and, concurrently, preserving the precision of high-sensitivity measurements.